The present invention relates generally to time-of-use watt-hour meters and, more particularly, to time-of-use meters that retain a preselected number of energy demand values.
Time-of-use watt-hour meters provide electric utilities and their customers with the capability for utilizing complex rate structures. When time-of-use watt-hour meters are used, different billing rates can be used for different times of the day, days of the week and for other special events, such as holidays and season changes. In order to operate as a time-of-use meter, a watt-hour meter must have the capability for measuring time and maintaining clock and calendar values. The cost of electrical energy consumption can then be determined as a function of the particular time when the electrical energy is consumed.
Demand watt-hour meters serve a slightly different function. They permit electric utilities to determine their customers' bills according to the customer's peak energy demand over a prescribed length of time, such as a billing period. Although each customer's utility bill will vary as a function of the total energy consumed over a prescribed billing period, the electric bill will also be determined as a function of the customer's highest demand for electrical energy during that billing period. The purpose of this type of billing structure is to charge the electrical consumer according to the power capabilities, on the part of the electrical utility, that that consumer necessitates. For example, if a particular electrical consumer experiences short periods of extremely high electrical energy demand during a billing period, the electric utility must provide transformers and associated hardware that is sufficient for the customer's peak energy requirements.
There are various known ways by which an electric utility can measure a consumer's demand for electrical energy. One way is to provide the electrical energy consumer with a recording device that continuously measures and records the level of the consumer's energy consumption. At the end of a prescribed period of time, the electric utility would then read a memory medium, such as a magnetic tape or solid-state memory device, and determine the time and magnitude of that customer's peak demand for electrical energy. Apparatus which provides this type of demand profile for a customer is, by its nature, memory intensive. Time-of-use demand meters, which are typically not of the mass data recording type, are generally limited in capability to storing a single demand value which represents the highest single period of peak energy demand of a particular electric consumer.
More recently, time-of-use watt-hour meters have been designed to incorporate a microprocessor. The incorporation of a microprocessor enables the time-of-use watt-hour meter to determine and store a plurality of peak demand values for a particular consumer. Since a typical watt-hour meter has severe spatial limitations and high reliability requirements, they are designed to utilize a minimum number of electronic components and, therefore, available memory storage is necessarily limited. Because of this limitation, watt-hour meters that are configured to be used as demand meters typically store three or less demand levels for a particular billing period. These demand levels represent the maximum demand for each rate period which is a simplified historic profile of the consumer's electric energy requirements.
When measuring electrical demand, care must be taken to avoid various situations that can cause misleading information. For example, the consumer's peak energy demand must, of course, be measured over some finite period of time. This period of time must be great enough to avoid being unduly influenced by a solitary energy spike of very short duration. Otherwise, the consumer would be unjustly billed for a peak demand level that occurred only once and for a very short period of time, such as during the startup of a large motor. However, if the peak demand levels are measured over a much longer period of time, peak demand will be incorrectly under-recorded. For example, if a consumer experiences a high electrical demand that lasts for 20 minutes and the demand meter uses a time base of 2 hours, the apparent demand will be much lower than the actual demand. Therefore, in order to record a truly representative demand level, a reasonably short period of time should be used. Historically, a 15-minute interval has been used in order to determine a consumer's peak demand over a 15-minute period.
The use of demand intervals, as discussed above, can also cause another misleading situation. If the consumer has accurate knowledge of the beginnings and endings of the demand intervals, the consumption of electric energy can be timed so as to advantageously span two demand intervals. For example, assuming the consumer has knowledge that a high energy consumption will last 15 minutes and the demand intervals are 15 minutes long, this energy consumption can be timed to begin midway through one demand interval and end midway through the following interval. The effect of this procedure would be to make the apparent demand be significantly less than the actual demand and the electric utility would therefore underbill the consumer. To avoid this strategy, electric utilities sometimes employ a sliding demand interval which is known to those skilled in the art and which will be described in greater detail below. Another problem that typically occurs during the recording of demand information is that the value is recorded with no accompanying indication of its time or date of occurrence. Because of this, neither the electric utility nor the electric energy consumer can properly correlate the demand to a consumption activity.
The present invention provides a time-of-use watt-hour meter that is capable of recording the maximum demand value, its time and date for each rate period and a plurality of individual interval demands with time and date of occurrence of electrical energy consumption which, in a preferred embodiment, is equal to five demands. Furthermore, the present invention is able to discriminate between demand levels that occur within a preselected period of time.
A time-of-use watt-hour meter made in accordance with the present invention comprises a means for measuring electrical energy consumption. Many such measuring means are known to those skilled in the art of electrical measurement. Watt-hour meters of many types are in common use in many diverse applications. Typically, electrical usage is indicated by the rotation of a flat disk and the movement of a plurality of register dial indicators. A metering system for electric utility power line measurements is disclosed in U.S. Pat. No. 4,077,061 which issued to Johnston et al. on Feb. 28, 1978. It incorporates a system sequence controller and calculator that provide programmed control for processing digital control and data signals and for producing digital calculations of electrical energy parameters from the binary representation of the instantaneous signal values. Memory registers are used to total and accumulate digitally calculated values for producing visual displays and output signals that correspond to electric energy parameters that are being measured. A multiple rate meter for totalizing electrical power usage from an alternating current supply is disclosed in U.S. Pat. No. 4,081,746 which issued to Snyder et al. on Mar. 28, 1978. It includes a rotatable member that rotates at a speed which is proportional to the rate of electrical energy usage. Solid-state circuitry is used to time the speed of a rotatable member versus a supply frequency. It provides a multiple rate meter which provides a signal for operating a small electrical solenoid operatively connected to change registers, add a register or change the gear ratios in the drive train of a single register. In a watt-hour meter made in accordance with the present invention, some means for generating pulses, as a function of energy consumption, is used to provide a signal that represents the amount of electrical energy being consumed. A convertible multi-rate dial register for energy consumption meters is disclosed in U.S. Pat. No. 4,365,194 which issued to Halstead et al. on Dec. 21, 1982 and a demand meter for on-peak maximum demand metering is disclosed in U.S. Pat. No. 3,913,014 which issued to Halstead et al. on Oct. 14, 1975.
Various means are known to those skilled in the art for initiating pulses that are representative of the consumption of electrical energy. An opto-electronic pulse initiator for producing meter data pulses from a meter having a rotating movement is disclosed in U.S. Pat. No. 4,034,292 which issued to McClelland on July 5, 1977. A photoelectric pulse initiator that includes a rotating pattern having a single track of indicia scanned by two opto-electronic sensors is disclosed in U.S. Pat. No. 4,047,024 which issued to Henderson on Sept. 6, 1977. U.S. Pat. No. 3,878,391, which issued to McClelland et al. on Apr. 15, 1975, discloses a radiometric pulse initiator for 3-wire remote meter telemetry systems that includes interchangeable reflective pattern drums that have alternative preselectable numbers of reflecting segments for generating desired pulse rate in response to a predetermined amount of rotation of a watt-hour meter movement. Many such pulse initiating devices are known to those skilled in the art and the present invention is not dependent upon one particular pulse initation technique, but can be used in association with any one of a number of devices that are capable of producing a series of pulses where each of the pulses represents a predetermined quantum of electrical energy consumption.
A programmable watt-hour energy meter having a radiation responsive external data interface is disclosed in U.S. Pat. No. 4,298,839 which issued to Johnston on Nov. 3, 1981. It provides an externally programmable time-based measuring system that is disposed in a sealed enclosure for mounting the watt-hour meter at a particular metering location. A radiation sensitive external data interface receives and transmits data encoded radiations through a transparent communications window of the enclosure. This general type of watt-hour meter is suitable for use in association with the present invention. A particular style of time-of-use watt-hour meter is disclosed in U.S. patent application Ser. No. 292,978 which was filed on Aug. 14, 1981 by Benbow. This particular style of time-of-use meter mounts an electronic register directly to a transparent cover of a time-of-use meter.
The present invention relates directly to both demand meters and time-of-use meters. It provides a watt-hour meter that is capable of recording and storing a plurality of demand levels along with the time and date that each of the plurality of demand levels occurred. It also provides a means for avoiding the disadvantageous result that can occur when a sustained period of high demand extends over a plurality of these demand intervals.
A problem that can occur when recording a plurality of demands for a particular consumer is that the demand intervals may be short enough in duration for a sustained period of relatively high energy demand to encompass a plurality of demand intervals. This situation tends to reduce the informational value of the plurality of peak demand levels available to the utility and the consumer. For example, if a demand meter utilizes 15-minute demand intervals, a period of high electrical energy consumption that lasts 75 minutes can provide the five highest 15-minute interval demands during a single continuous time period. The result of this situation would be that the electric utility would obtain information, concerning this particular consumer, that indicates only one long period of high electrical consumption during the billing period. This would be indicated by five demand intervals which are contiguous. Although this information is accurate, it is redundant. What may actually have happened is that the consumer experienced many other periods of high electrical energy consumption that were only slightly less than the five indicated demand intervals. However, since none of the other periods of high electrical consumption were as high as the five contiguous demand intervals, they were not reported and neither the consumer nor the electric utility would be informed of their existence. Since these other periods of high electrical energy demand may be important to both the consumer and the utility, it would be advantageous if a demand meter was provided with an ability to distinguish between demand intervals which are contiguous and take steps to avoid this possibly misleading information. As discussed above, this disadvantageous circumstance can result when all of the plurality of demand levels are contiguous. Although the stored data is accurate, it can be misleading in that all of the demand levels are related to a single sustained period of high electrical energy consumption.
The present invention provides a means for defining a survey period which lasts a preselected amount of time. Another period of time, referred to herein as a demand interval, is defined and used as a precise quantum of time during which electrical energy consumption is measured. In a preferred embodiment of the present invention, the demand interval is less than the survey period. Furthermore, the length of the survey period can be chosen to be an integral multiple of the demand interval. It should be understood that this time relationship between the survey period and the damand interval is not a requirement of the present invention and other relationships between these two time periods can be used in a watt-hour meter made in accordance with the present invention. For example, an alternate embodiment of the present invention could utilize a survey period that is actually shorter than a demand interval.
A watt-hour meter made in accordance with the present invention also comprises a means for measuring electrical energy consumption. As discussed above, this type of measuring means can be any one of a number of electrical metering devices known to those skilled in the art. A means for measuring time is also provided so that the survey period and demand interval can be accurately initiated and terminated. The time measuring means enables the present invention to maintain a calendar and clock so that the exact time and date of energy demand peaks can be recorded and stored. In a preferred embodiment of the present invention, a 60 Hz signal is used to provide a series of timing signals when alternating current power is available. When running on battery power, the timing pulses occur every four seconds in order to reduce the energy drain on a backup battery system.
The present invention also provides a means for selecting the highest interval demand that occurs during each survey period. This highest interval demand is stored, along with the highest interval demands of successive survey periods and a means is provided for comparing each of these highest interval demands to a plurality of stored maximum demands that have previously occurred. Over a sustained period of time, this process results in the storing of the five highest interval demands that have occurred up to the present period of time. This plurality of highest interval demands is maintained and stored during a preselected time interval, such as a billing period.
A microprocessor is used in a preferred embodiment of the present invention to define the survey periods and demand intervals. The microprocessor is also configured to record and store the measured electrical energy consumption that occurs during each demand interval and compare the values to determine and store a preselected number, such as five, of the highest interval demands occurring during a sustained period of time.
A random access memory (RAM) device is associated with the microprocessor of the present invention in order to provide storage of pertinent data and a means for communicating this data to an external device, such as a portable programmer/reader, is also provided. In a preferred embodiment of the present invention, a visual display is used so that an operator can visually determine the values of the preselected number of highest interval demands that have occurred as of any point in time.